Move event-loop configury to common.m4

Move event-loop configury to common.m4

gdb_select.h and the event loop require some configure checks, so this
moves the needed checks to common.m4 and updates the configure
scripts.

gdb/ChangeLog
2020-04-13  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* configure.ac: Remove checks that are now in GDB_AC_COMMON.

gdbserver/ChangeLog
2020-04-13  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* config.in: Rebuild.

gdbsupport/ChangeLog
2020-04-13  Tom Tromey  <tom@tromey.com>

	* config.in, configure: Rebuild.
	* common.m4 (GDB_AC_COMMON): Check for poll.h, sys/poll.h,
	sys/select.h, and poll.